Apparatuses are known which are used for the filtration of liquids and which comprise, within a casing, a set of filtering elements made of porous material which are disposed over a certain length of the casing in an axial direction and which occupy a substantial part of the cross-section of the casing. A conduit for the admission of liquid to be filtered, as well as a conduit for the evacuation of the liquid after filtration, open into the casing in zones situated on both sides of the zone occupied, along the axial direction of the casing, by the filtering elements.
The casing comprises a cover, on which the conduit for the evacuation of filtered liquid is generally fixed, this cover giving access to the set of filtering elements, for example for the purpose of carrying out replacement thereof.
During the operation of the filtration apparatus, the filtering elements made of porous material retain the impurities by the fluid, and these impurities progressively fill the interstices of the porous material.
The loss of head through the filtering elements increases, in the course of the utilization of the filtration apparatus. In order to maintain satisfactory conditions of operation of the filtration apparatus, it is necessary to regenerate the filtering elements after a certain operating time.
A first method of regeneration consist in replaceing all the filtering elements when the loss of head through these elements has exceeded a specified limiting value, for a given throughput.
This method involves disadvantages, because it necessitates the demounting of a part of the apparatus and because it involves the handling of a large volume of waste materials, comprising to all the impurities retained by the cleaned filtering elements and these filtering elements themselves.
This leads to increased labor cost and an increased period of non-availability of the filtration apparatus; furthermore, it may be necessary to handle and to process large quantities of waste materials, after cleaning of the filter.
Additional difficulties arise when the filtration apparatuses of the type described hereinabove are utilized for the processing of fluids containing impurities consisting of dangerous materials. This is so, for example, in the case of nuclear power stations, when the filtration apparatuses such as are described are utilised for the processing of a fluid transporting radioactive materials, for example for the processing of the cooling fluid of the reactor. In this case, the personnel entrusted with the replacement of the filtering elements must ensure the maintenance of a set having a large volume, enclosing a large quantity of radioactive material. Such personnel is thus exposed to ionising radiations and may be contaminated in the course of the maintenance.
A second process for the regeneration of the filtering elements of a filtration apparatus consists in isolating the apparatus from its normal circuit and, without demounting it, causing a certain quantity of clean fluid to pass in countercurrent through the filtering elements. These elements are then cleared of the particles retained during the operation of the apparatus. This method avoids the demounting of the filter and the replacement of the filtering elements. However, it only partially solves the problems associated with the recovery of waste materials during the operation of regeneration. These waste materials are, in fact, found again in the form of effluents consisting of the fluid utilized to carry out the cleaning of the filtering elements containing the waste materials separated from the filtering elements. The volume of fluid necessary to carry out the cleaning may be fairly large and, in the case of toxic or radioactive products, the effluents are contaminated and must be processed before the discharge thereof outside the site of the industrial installation in which the filtration apparatus is utilized. The volume of the effluents may be very large; this necessitates the utilization of costly processing stations.
In order to avoid the disadvantages set forth hereinabove, processes for the regeneration of the filtration apparatuses have been proposed which consist in cleaning the filtering elements, by means of ultrasonic waves, within the filter body itself. The ultrasonic waves are emitted within the fluid filling the filter body, in such a manner as to reach the filtering elements and to ensure the extraction of the impurities retained by these filtering elements. The impurities are collected at the lower part of the filter and may be evacuated through an evacuation conduit. The emission of ultrasonic waves within the fluid filling the filter body creates an acoustic pressure field. If the amplitude of this pressure field is sufficient, this results in a phenomenon of cavitation; for this to be the case, it is sufficient for the average underpressures created within the liquid to be at least equal to the pressure difference between the actual pressure and the vapor pressure of the liquid at the temperature considered.
This phenomenon of cavitation involving the formation of bubbles of steam may be maintained by the acoustic field. The shock waves, as well as the turbulent microcurrents which result therefrom, permit the extraction of the particles filtered and collected by the porous medium.
This cleaning operation does not necessitate the demounting or modification of the filter; the inlet and outlet fluid pipes connected to the filter body do not need to be isolated from the operating circuit except for a limited vapor.
French Certificate of Utility No. 2,421,661 discloses a process and an installation for cleaning a filter of the type described hereinabove utilizing transducers emitting ultrasonic waves which may be disposed within the wall of the filter body itself. However, the transducers used are disposed at one end of the filter body and generally on the cover. This result in an unsatisfactory operation, in the case where the acoustic emission has to affect the volume of a filtration assembly consisting of elongate filtering elements placed along the axial direction of the casing.